Flow-induced grating from cholesteric mixtures

In this work we present a new technique for obtaining large diffraction gratings (some cm) by means of a simple filling of cells having a planar treatment of their inner surfaces. A homogeneous mixture, composed of a cholesteric liquid crystal and a nematic liquid crystal monomer, was used. During the filling process, the flow induces a phase separation between the cholesteric liquid crystal and the liquid crystal monomer and, at the same time, the latter is oriented planar to the surfaces of the cell. Phase separation produces alternate arrays constituted by the cholesteric liquid crystal and the nematic liquid crystal monomer. Successive UV polymerization of these films yields a permanent grating. We have investigated the transmitted and first order diffracted beam efficiency for films obtained at different temperatures. The morphology of the films was studied by using an optical microscope equipped with crossed polarizers and by electron microscopy in order to control the shape of the arrays and the alignment of the oriented polymer.     

Liquid crystal orientation in elliptic droplets in nematic emulsions

Elliptic droplets of nematic liquid crystal dispersed in a fluid organic monomer were obtained by phase separation from an isotropic mixture consisting of an organic monomer and a nematic liquid crystal contained in a poly(ethylene terephthalate) cell with inner surfaces treated with rubbed polyimide. The elliptic shape is a consequence of the constraint upon droplet growth along the direction perpendicular to the cell surfaces owing to the small thickness. Then, the resulting droplets will have a contact area with the inner surfaces of the cell treated with polyimide, which will impart a planar orientation on the liquid crystal in the droplet. By means of an optical microscope, using a simple pin hole of 5 μm, we have selected single droplets for a series of samples having different contact areas. By polarized infrared spectroscopy we have also studied the liquid crystal orientation in selected areas of the droplets. We then report the dependence of the order parameter of the liquid crystal on different contact areas with the alignment surface of the cell. The good degree of planar alignment of the liquid crystal in the elliptic droplets allows the use of such a technique for realizing electro-optical films operating in the reverse mode. We report the electro-optical transmission of reverse mode films with different sizes of elliptic droplet.     

Liquid crystal orientation in elliptic droplets in nematic emulsions

Elliptic droplets of nematic liquid crystal dispersed in a fluid organic monomer were obtained by phase separation from an isotropic mixture consisting of an organic monomer and a nematic liquid crystal contained in a poly(ethylene terephthalate) cell with inner surfaces treated with rubbed polyimide. The elliptic shape is a consequence of the constraint upon droplet growth along the direction perpendicular to the cell surfaces owing to the small thickness. Then, the resulting droplets will have a contact area with the inner surfaces of the cell treated with polyimide, which will impart a planar orientation on the liquid crystal in the droplet. By means of an optical microscope, using a simple pin hole of 5 μ m, we have selected single droplets for a series of samples having different contact areas. By polarized infrared spectroscopy we have also studied the liquid crystal orientation in selected areas of the droplets. We then report the dependence of the order parameter of the liquid crystal on different contact areas with the alignment surface of the cell. The good degree of planar alignment of the liquid crystal in the elliptic droplets allows the use of such a technique for realizing electro-optical films operating in the reverse mode. We report the electro-optical transmission of reverse mode films with different sizes of elliptic droplet.     

Photo‐optical properties of polymer composites based on stretched porous polyethylene filled with photoactive cholesteric liquid crystal

A new type of polymer–liquid crystal composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene films were used as polymer matrices. To induce a cholesteric phase in a commercial nematic host, a chiral photochromic dopant based on sorbide and cinnamic acid capable of E–Z isomerization under UV irradiation was used. A merocianine‐type substance was selected as a dichroic dye. Introduction of a dye‐doped cholesteric mixture with a helical pitch higher than ～300 nm to polymer film led to an almost complete transition from a cholesteric to an oriented nematic phase, as well as to an increase in birefringence and the appearance of dichroism. Decrease of the helical pitch by increasing in the chiral dopant concentration in the liquid crystal–polymer composite results in a reduction of the dichroism values. UV irradiation of polymer composite leading to an isomerization of the chiral dopant and helix untwisting induces a noticeable gradual growth of dichroism and birefringence. These new composites can be considered as promising materials for optical applications.     

Dye induced great enhancement of broadband reflection from polymer stabilized cholesteric liquid crystals

A series of polymer stabilized cholesteric liquid crystals (PSCLCs) films were prepared from cholesteric liquid crystal (Ch‐LC) mixtures containing different components such as non‐reactive LC monomer, polymerizable monomer, chiral dopant, dye, and photoinitiator upon polymerization. The influence of the polymerizable monomer and dye of Ch‐LC mixtures on the reflection properties was investigated. The reflection bandwidth for all the samples can be increased by photo‐polymerization, and the network upon polymerization derived from two different polymerizable monomers with both one and two functional groups is more effective than that from one polymerizable monomer for broadening the reflection band. Especially, a dye‐doped Ch‐LC film can reflect incident light with the bandwidth over the wavelength range of 550–2350 nm, which is due to a greater pitch gradient formed inside of Ch‐LC film. The gradient pitch network structure was firstly demonstrated by scanning electron microscopy (SEM) with the film prepared from high diacrylate monomer concentration and subsequently proved by using a wash‐out/refill method. The nematic liquid crystals monomers was infiltrated into the polymer network that was prefabricated by removing the low molar weight LCs from the original PSCLCs film, and SEM exhibited the existence of a pitch gradient across the film thickness. The refilled nematic liquid crystals film showed broadband reflection after polymerzition, too. Copyright © 2011 John Wiley & Sons, Ltd.     

Photo-optical properties of polymer composites based on stretched porous polyethylene filled with photoactive cholesteric liquid crystal

A new type of polymer-liquid crystal composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene films were used as polymer matrices. To induce a cholesteric phase in a commercial nematic host, a chiral photochromic dopant based on sorbide and cinnamic acid capable of E-Z isomerization under UV irradiation was used. A merocianine-type substance was selected as a dichroic dye. Introduction of a dye-doped cholesteric mixture with a helical pitch higher than ∼300 nm to polymer film led to an almost complete transition from a cholesteric to an oriented nematic phase, as well as to an increase in birefringence and the appearance of dichroism. Decrease of the helical pitch by increasing in the chiral dopant concentration in the liquid crystal-polymer composite results in a reduction of the dichroism values. UV irradiation of polymer composite leading to an isomerization of the chiral dopant and helix untwisting induces a noticeable gradual growth of dichroism and birefringence. These new composites can be considered as promising materials for optical applications.     

Copolyesters exhibiting a thermotropic cholesteric phase

Mesomorphic copolymers were synthesized by incorporating varying ratios of azelaic acid and (+)-3-methyl adipic acid into copolyesters based upon 4,4′-dihydroxybiphenyl. Introduction of the phenylene (+)-3-methyl adipate unit broadens the temperature range of the nematic phase of the azelate homopolymer and, at the same time, produces a chiral nematic (cholesteric) mesophase which exhibits various iridescent colors. Circular dichroism measurements were used to determine the pitch of the cholesteric liquid crystals. The pitch decreased gradually with increasing temperature, and the inverse pitch increased in direct proportion to the molar content of the units containing (+)-3-methyl adipate. The twisted cholesteric structure could be conserved by quenching to produce films with various colors at room temperature. Annealing these films at a temperature immediately below that of the crystal–mesophase transition improved the regularity of the cholesteric structure.     

Induction of Helical Structure by Sesamin,and Production of Oriented Helical Polymer with Liquid Crystal Magneto‐Electrochemical Polymerization

Sesamin was employed as a chiral dopant for preparing cholesteric liquid crystals with right‐handed helical architecture. Helical twisting power of sesamin is to be 13.4 μm^?1. Electrochemical polymerizations were carried out with sesamin‐induced cholesteric liquid crystal electrolyte solution for obtaining conjugated polymer films with helical structure. The film was transcribed the helical order from the liquid crystal electrolyte solution with helical structure produced by sesamin during the polymerization process. The helical axes of the macromolecular superstructure of the polymer films were oriented in a magnetic field of 4.5 T. This results demonstrated liquid crystal magneto‐electrochemical polymerization with helical structure induced by sesamin as a natural chiral compound. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 1894–1899     

A Review of the Status of Polymerization in Thermotropic Liquid Crystal Media and Liquid Crystalline Monomers

Thermotropic liquid crystals offer uniquely ordered media for intermolecular reactions such as polymerization. Unlike the recently investigated topochemical polymerizations where molecules are rigidly constrained on lattice points [l], the liquid crystalline state permits full two-dimensional (nematic and cholesteric) and one-dimensional (smectic) movement. In addition, various degrees of translational freedom are attainable depending on the class of mesogen. Such freedom of choice, plus the ability to orient nematic and cholesteric mesogens on a molecular scale in an electric or magnetic field or on certain surfaces, makes the liquid crystal state an attractive polymerization medium.     

Rough surfaces for orientation control in reverse mode polymer dispersed liquid crystal films

Reverse mode operation shutters have been achieved with polymer dispersed liquid crystals by means of polymerization-induced phase separation of nematic mixtures consisting of a low molecular mass liquid crystal and a liquid crystalline monomer. Fluid mixtures were homeotropically aligned by rough surfaces and transparent films were obtained after polymerization. Transmittance in the OFF state can be larger than 80% and decreases to less than 1% when an electric field of about 2 V μm^-1 at 1 kHz is applied (ON state). Both rise and decay times can be lower than 10 ms and the drop in the OFF state normal transmittance is drastically reduced with respect to conventional polymer dispersed liquid crystals since samples exhibit a reverse morphology. The role played by surface roughness is also discussed.     

Polymer-stabilized cholesteric liquid crystal microgratings: a comparison of polymer network formation and electro-optic properties for mesogenic and non-mesogenic monomers

Polymer-stabilized liquid crystal microgratings are made using a focused Gaussian UV laser beam to photopolymerize 3 wt % reactive monomer in a cholesteric liquid crystal host. In a typical case, round gratings of 300 μm diameter and 10 μm pitch are produced. The microgratings highlight interesting differences between mesogenic and non-mesogenic monomers in the assembly and spatial distribution of polymer networks formed in a cholesteric host. We also observe a corresponding variation in the electro-optical properties of the stabilized gratings. In the mesogenic case, the grating state of the liquid crystal is faithfully captured even for relatively short UV exposures and over regions only a few pitch lengths in size. These findings are consistent with phase separation of the mesogenic monomer into regular domains templated by periodic, macroscopic variations in orientational order of the host. This templating effect is significantly reduced in the non-mesogenic case.     

UV tuning of the electro-optical and morphology properties in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro-optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro-optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro-optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro-optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro-optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.     

A study of the transition of liquid-crystal alignment from homeotropic to planar on a polyimide layer

The alignment of nematic liquid crystals by rubbed polyimide surfaces has been well-studied and developed. A novel polyimide film which induced a homeotropic alignment of the nematic liquid crystal without rubbing or with weak rubbing strength was presented. However, there was a transition from homeotropic to planar alignment of the nematic liquid crystal after strong rubbing. In order to study the transition, the polyimide surface was investigated by atomic force microscopy, surface free energy measurement and angle-resolved analysis X-ray photo-electron spectroscopy before and after rubbing with a velvet fabric. It was found that both the change of surface polarity and surface morphology were not the reasons for the transition. The droop of the side chain on the polyimide surface after the rubbing treatment was detected by angle-resolved analysis X-ray photo-electron spectroscopy. Owing to the special structure of the novel polyimide, the X-ray photo-electron spectroscopy was successfully used for the first time to analyse the conformational change of the side chain of a polymer. In conclusion, the transition of nematic liquid crystal alignment from homeotropic to planar after rubbing was influenced by the side chain conformation of the polyimide.     

Polymer-stabilized cholesteric liquid crystal microgratings: a comparison of polymer network formation and electro-optic properties for mesogenic and non-mesogenic monomers

Polymer-stabilized liquid crystal microgratings are made using a focused Gaussian UV laser beam to photopolymerize 3 wt % reactive monomer in a cholesteric liquid crystal host. In a typical case, round gratings of 300 μm diameter and 10 μm pitch are produced. The microgratings highlight interesting differences between mesogenic and non-mesogenic monomers in the assembly and spatial distribution of polymer networks formed in a cholesteric host. We also observe a corresponding variation in the electro-optical properties of the stabilized gratings. In the mesogenic case, the grating state of the liquid crystal is faithfully captured even for relatively short UV exposures and over regions only a few pitch lengths in size. These findings are consistent with phase separation of the mesogenic monomer into regular domains templated by periodic, macroscopic variations in orientational order of the host. This templating effect is significantly reduced in the non-mesogenic case.     

Effect of specific rotation of chiral dopant and polymerization temperature on reflectance properties of polymer stabilized cholesteric liquid crystal cells

A series of polymer stabilized cholesteric liquid crystal (PSCLC) cells were prepared by photo‐polymerization of a cholesteric liquid crystal (Ch‐LC) mixture containing a nonreactive LC, a nematic diacrylate and a novel cholesteryl monomer. The influence of the specific rotation and concentration of the chiral dopants, and the polymerization temperature on reflection properties was investigated. The results demonstrate that the reflection band was broadened after polymerization for all the systems both left‐handed S811 and right‐handed R1011 as the chiral dopant, which is speculated to be a result of an inhomogeneous consumption of the chiral monomer within the system. Additionally, the polymer temperature plays an integral role in the observed reflection spectra, and at optimum polymerization temperature the broadband reflection effect becomes much more pronounced. Scanning electron microscopy (SEM) was used to examine the role of microscopic changes of the polymer network induced by polymerization temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1562–1570, 2008     

Comparative study of holographic recording in cholesteric and nematic azo-containing side-chain polymers

For the first time a comparative study of holographic recording in planarly oriented films of nematic and cholesteric azobenzene-containing polymers was performed. The influence of temperature and light intensity on the values of diffraction efficiencies of holographic gratings was investigated. The kinetics of grating relaxation at different temperatures was studied. It was shown that the helical supramolecular structure of cholesteric copolymer causes a significant decrease of the diffraction efficiency in comparison with the one observed for the nematic state of the homopolymer.     

Influence of a Change in Helical Twisting Power of Photoresponsive Chiral Dopants on Rotational Manipulation of Micro‐Objects on the Surface of Chiral Nematic Liquid Crystalline Films

Herein we report a group of five planar chiral molecules as photon‐mode chiral switches for the reversible control of the self‐assembled superstructures of doped chiral nematic liquid crystals. The chiral switches are composed of an asymmetrically substituted aromatic moiety and a photoisomerizing azobenzene unit connected in a cyclic manner through methylene spacers of varying lengths. All the molecules show conformational restriction in the rotation of the asymmetrically substituted aromatic moiety in both the E and Z states of the azobenzene units resulting in planar chirality with separable enantiomers. Our newly synthesized compounds in pure enantiomeric form show high helical twisting power (HTP) in addition to an improved change in HTP between the E and Z states. The molecule with a diphenylnaphthalene unit shows the highest ever known initial helical twisting power among chiral dopants with planar chirality. In addition to the reversible tuning of reflection colors, we employed the enantiomers of these five compounds in combination with four nematic liquid crystalline hosts to study their properties as molecular machines; the change in HTP of the chiral dopant upon photoisomerization induces rotation of the texture of the liquid crystal surfaces. Importantly, this study has revealed a linear dependence of the ratio of the difference between HTPs before and after irradiation against the absolute value of the initial HTP, not the absolute value of the change in helical twisting power between two states, on the angle of rotation of micro‐objects on chiral nematic liquid crystalline films. This study has also revealed that a change in irradiation intensity does not affect the maximum angle of rotation, but it does affect the speed of rotational reorganization of the cholesteric helix.     

Morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals

We have investigated the morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals as a function of liquid crystal loading. Reverse mode shutters have been obtained by a polymerization-induced phase separation of mixtures, consisting of a liquid crystalline monomer and a non-reactive nematic liquid crystal, placed between rough conductive surfaces. Such surfaces are able to keep the photopolymerizable mixtures homeotropically aligned without the use of any aligning polymer substrate. OFF state transmittances are always larger than 80% and the switching fields decrease if the non-reactive liquid crystal percentage is increased. Both rise and decay times are always lower than 10 ms. The electro-optical properties have been related to the sample morphology and a simple mode is proposed.     

Morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals

We have investigated the morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals as a function of liquid crystal loading. Reverse mode shutters have been obtained by a polymerization-induced phase separation of mixtures, consisting of a liquid crystalline monomer and a non-reactive nematic liquid crystal, placed between rough conductive surfaces. Such surfaces are able to keep the photopolymerizable mixtures homeotropically aligned without the use of any aligning polymer substrate. OFF state transmittances are always larger than 80% and the switching fields decrease if the non-reactive liquid crystal percentage is increased. Both rise and decay times are always lower than 10 ms. The electro-optical properties have been related to the sample morphology and a simple mode is proposed.     

基于胆甾相液晶的可调制光子晶体

将胆甾相液晶填充进胶体晶体内部空隙, 通过胆甾相液晶与胶体晶体的耦合, 构建了一种新型可调制液晶光子晶体. 填充于胶体晶体内部的胆甾相液晶织构呈现典型的手性近晶相(S)特征. 由于胆甾相液晶具有特定的选择性反射, 当胶体晶体的带隙处于胆甾相液晶的反射波长范围之内, 则随着温度的改变, 胶体晶体的带隙与胆甾相液晶的带隙同时发生蓝移. 在一定温度条件下, 胆甾相液晶的带隙将与胶体晶体的带隙发生耦合, 实现了光子晶体带隙在单峰与双峰之间的可逆切换.